High octane unleaded aviation gasolines

ABSTRACT

Novel aviation fuel compositions contain a substantially positive or synergistic combination of an alkyl tertiary butyl ether, an aromatic amine and, optionally, a manganese component. The basefuel containing the additive combination may be a wide boiling range alkylate basefuel.

BACKGROUND OF THE INVENTION

[0001] The invention relates generally to aviation gasoline (Avgas)compositions and methods of making and using such compositions. Moreparticularly, the present invention concerns high octane Avgascompositions containing a non-leaded additive package and methods ofmaking and using such compositions.

[0002] Conventional aviation gasoline (Avgas) generally contains anaviation alkylate basefuel and a lead-based additive package. Theindustry standard Avgas known as 100 Low Lead (100 LL) contains the leadadditive tetraethyllead (TEL) for boosting the anti-knock property ofthe Avgas over the inherent anti-knock property of its aviation alkylatebasefuel. Knocking is a condition of piston-driven aviation engines dueto autoignition, the spontaneous ignition of endgases (gases trappedbetween the cylinder wall and the approaching flame front) in an enginecylinder after the sparkplug fires. A standard test that has beenapplied to measure the anti-knock property of lead-based Avgas undervarious conditions is the motor octane number (MON) rating test (ASTMD2700). Another standard test applied to lead-based Avgas is thesupercharge (performance number) rating test (ASTM D909).

[0003] Despite the ability of lead-based Avgas to provide goodanti-knock property under the severe demands of piston-driven aviationengines, such lead-based compositions are meeting stricter regulationsdue to their lead and lead oxide emissions. Current U.S. regulations seta maximum amount of TEL for aviation fuels at 4.0 ml/gal and concernsfor the negative environmental and health impact of lead and lead oxideemissions may effect further restrictions.

[0004] Gaughan (PCT/US94/04985, U.S. Pat. No. 5,470,358) refers to ano-lead Avgas containing an aviation basefuel and an aromatic amineadditive. The Avgas compositions exemplified in Gaughan reportedlycontain an aviation basefuel (e.g., isopentane, alkylate and toluene)having a MON of 92.6 and an alkyl- or halogen-substituted phenylaminethat boosts the MON to at least about 98. Gaughan also refers to othernon-lead octane boosters such as benzene, toluene, xylene, methyltertiary butyl ether, ethanol, ethyl tertiary butyl ether,methylcyclopentadienyl manganese tricarbonyl and iron pentacarbonyl, butdiscourages their use in combination with an aromatic amine because,according to Gaughan, such additives are not capable by themselves ofboosting the MON to the 98 level. Gaughan concludes that there is littleeconomic incentive to combine aromatic amines with such other additivesbecause they would have only a very slight incremental effect at the 98MON level.

[0005] It would be desirable to find alternative Avgas compositions thatavoid the use of lead-based additives and have good performance inpiston-driven aviation engines. It would also be desirable to find Avgascompositions that could use less expensive bassefuels.

SUMMARY OF THE INVENTION

[0006] The Avgas compositions of the invention contain a combination ofnon-lead additives (also referred to as the “additive package”)including an alkyl tertiary butyl ether and an aromatic amine. Theadditive package may further include manganese, for example, as providedby methyl cyclopentadienyl manganese tricarbonyl (MMT). In a preferredembodiment, the substantially positive or synergistic additive packageis combined with a wide boiling range alkylate basefuel. In a furtherpreferred embodiment, the inventive Avgas composition is an unleadedAvgas having good performance in a piston-driven aviation engine asdetermined by one or more ratings including MON, Supercharge and KnockCycles/Intensity at maximum potential knock conditions of an aviationengine.

[0007] The invention is also directed to a method of making an unleadedAvgas composition wherein the additive package is combined with abasefuel, such as a wide boiling range alkylate. The concentration ofthe additives in the Avgas may be based on a non-linear model, whereinthe combination of additives has a substantially positive or synergisticeffect on the performance of the unleaded Avgas composition. Theinvention is further directed to a method of improving aviation engineperformance by operating a piston-driven aviation engine with such Avgascompositions.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0008] For purposes of the invention, “Avgas” or “Avgas composition”refers to an aviation gasoline. In general, an Avgas is made of abasefuel and one or more additives.

[0009] The compositions according to the invention contain a combinationof additives including an alkyl tertiary butyl ether and an aromaticamine. The combination may further include a manganese component that iscompatible with the other additives and the base fuel, for example, asprovided by the addition of methyl cyclopentadienyl manganesetricarbonyl (MMT). The combination of additives is also referred to as“the additive package.”

[0010] The alkyl tertiary butyl ether in the additive package ispreferably a C₁ to C₅ tertiary butyl ether and more preferably methyltertiary butyl ether (MTBE) or ethyl tertiary butyl ether (ETBE). Thiscomponent of the additive package is also broadly referred to as theoxygenate.

[0011] The aromatic amine in the additive package is preferably of theformula:

[0012] where R₁, R₂, R₃ and R₄ are individually hydrogen or a C₁-C₅alkyl group. In a preferred embodiment, the aromatic amine additive isaniline, n-methyl aniline, n-ethyl aniline, m-toluidine, p-toluidine,3,5-dimethyl aniline, 4-ethyl aniline or 4-n-butyl aniline.

[0013] Methyl cyclopentadienyl manganese tricarbonyl (MMT) may also beincluded in the additive package, particularly to provide a magnesiumcomponent to the additive package.

[0014] The inventive Avgas compositions preferably comprise 0.1 to 40vol % alkyl tertiary butyl ether, 0.1 to 10 wt % aromatic amine and 0 to0.5 g manganese. For example, the inventive composition may comprise 15to 32 vol % methyl tertiary butyl ether, 1.5 to 6 wt % aniline and 0 to0.1 g manganese.

[0015] In a preferred embodiment, the additive package has asubstantially positive or synergistic effect in the Avgas composition towhich it is added. For purposes of this specification, the term“substantially positive,” in the context of the additive package, meansthat a successive additive that is added to the Avgas compositionsubstantially boosts the performance of the Avgas composition. In thecase of MON, “substantially positive” effect means that each successiveadditive boosts the Avgas MON, preferably by 0.5, more preferably by 1.0and most preferably by 1.5. For example, an Avgas containing a wideboiling range alkylate having a MON of 91.5 and an additive of 10 wt %aniline has a MON of 97.6. When that Avgas further contains a 40 vol %ETBE, the Avgas MON is boosted to 101.1. Such a composition contains asubstantially positive combination of additives because the overall MONof 101.1 is greater than the individual MON levels of 97.6 (10 wt %aniline) and 96.2 (40 vol % ETBE) and the addition of 40 vol % ETBEboosted the MON of the basefuel/10 wt % aniline composition by 3.5.

[0016] For purposes of this specification, the term “synergistic,” inthe context of the additive package, means that the effect of thecombined additives is greater than the sum of the performance achievedby the individual additives under the same conditions. In the case ofMON, synergistic means that the increase in MON due to the additivepackage is greater than the sum of MON increases for each additive whenit is the sole additive in the basefuel.

[0017] These definitions of “substantially positive” and “synergistic”effect are further understood in view of the numerous combinations ofadditives that result only in antagonistic combinations, wherein theoverall MON does not increased or decreases with the addition of otheradditives.

[0018] Combining multiple additives into a package that includes anaromatic amine has been viewed as an undesirable approach to improve theanti-knock property of an Avgas. (See Background of the Invention,Gaughan.) As further shown in the following Table 1, random mixtures ofmultiple octane boosting additives can result in antagonistic octaneeffects. TABLE 1 Non-linear Blending Octane Effects (Basefuel is wideboiling range alkylate.) Blend # ETBE (vol. %) Mn (g/gal) Aniline (wt.%) MON 1  0 0 10 97.6 2 40 0  0 96.2 3 40 0 10 101.1 4 40 0.5 10 97.9

[0019] As seen in Blend #4, the combination of basefuel/10% wtaniline/40 vol % ETBE/0.5 g/gal manganese results in an antagonisticeffect wherein the additive package (40 vol % ETBE/0.5 g/gal Mn/10 wt %aniline) does not boost the MON beyond that of the basefuel to anysignificant extent. Indeed, this additive package reduces the MONboosting effect of the basefuel/10% wt aniline/40% vol ETBE composition.

[0020] In a preferred embodiment, the additive package is combined witha basefuel containing a wide boiling range alkylate. Under thisembodiment of the invention, an Avgas can be made with a basefuel notconventionally used for Avgas. Under aviation standards (ASTM D-910),the basefuel in an Avgas is an aviation alkylate, which is a speciallyfractionated hydrocarbon mixture having a relatively narrow range ofboiling points. The inventive additive package may be added to anysuitable basefuel wherein the resulting combination of additive packageand basefuel is suitable for use as an Avgas, as based on performancecharacteristics and ratings and not necessarily on ASTM standards. Suchbasefuels include conventional aviation alkylates (e.g. within thespecifications of ASTM-910, including specifications for boiling pointsand distillation temperatures) and wide boiling range basefuels.

[0021] For purposes of this specification, the term “wide boiling rangealkylate” is defined as an alkylate containing components having a rangeof boiling points that is substantially wider than the range of boilingpoints in an aviation alkylate basefuel. Preferably, the wide boilingrange alkylate contains hydrocarbons having a range of boiling points upto at least about 350° F. More preferably, the boiling range is fromabout 85° F.±10° F. to about 400° F.±15° F. (which essentiallycorresponds to an automotive gasoline basefuel). The following Table 2provides an example of an aviation alkylate and a wide boiling rangealkylate. TABLE 2 Comparison of Wide boiling Range Alkylate and AviationAlkylate Fuels. Wide boiling range alkylate Wide boiling DistillationAviation range Aviation Tests Results Alkylate Tests alkylate AlkylateIBP*  88.1° F.  97.7° F. API 71.5 73.0 10% 147.9 155.3 RVP 7.6 psi 6.5psi 20% 179.4 178.5 Paraffins 99.2 vol. % 99.4 vol. % 30% 199.2 195.8Olefins 0.2 vol. % 0.4 vol. % 40% 209.8 206.0 Aromatics 0.6 vol. % 0.2vol. % 50% 216.6 212.1 MON 91.4 93.9 60% 222.4 215.7 RON 93.4 97.1 70%228.7 218.6 Perf.No. 85.4 97.4 80% 238.6 221.3 90% 262.9 224.9 FBP*397.2 233.4

[0022] The lower octane of the wide boiling range alkylate compared tothe aviation alkylate is due primarily to lower amounts of inherentlyhigh octane hydrocarbons, isopentane and isooctane, as well as higheramounts of higher molecular weight, higher boiling paraffins. Table 3presents gas chromatographic analyses of the aviation industry standard100 Low Lead, which uses aviation alkylate as the primary base stock(e.g., at least 88% vol) and the wide boiling range alkylate anddemonstrates the lower concentrations of isopentane and the isooctaneisomers in the wide boiling range alkylate. TABLE 3 Comparison of WideBoiling Range Alkylate and 100 Low Lead Concentration in Concentrationin Wide Boiling Range Alkylate 100 Low Lead (wt %) (wt %) Isopentane9.26 5.04 2,2,4- 30.93 21.89 trimethylpentane 2,2,3- 1.06 1.40trimethylpentane 2,3,4- 9.91 10.99 trimethylpentane

[0023] The distillation curve temperatures for the second half of thewide boiling range alkylate are considerably higher than the aviationalkylate because of the higher molecular weight paraffinic hydrocarbonspresent in the former.

[0024] A common result of having a higher concentration of largerparaffins, particularly with the straight chain or normal paraffins, isa lower octane value. The larger paraffin molecules present in the wideboiling range alkylate typically undergo more and faster isomerizationchemical reaction steps during the low temperature portion of theoxidation chemistry leading to auto-ignition. Isomerization steps inparaffin chemistry are very fast routes to free radical propagation andsubsequent autoignition. The oxidation steps leading to autoignitionbetween the two alkylate basefuels are different thus requiringdifferent fuel and additive formulations for optimal performance.Substituting high octane oxygenates for a substantial proportion of thealkylate basefuel reduces the number of rapid isomerization reactionsand replaces them with less reactive partial oxidation intermediates,thereby increasing the octane value of the fuel.

[0025] The preferred embodiment of the invention that uses the wideboiling range alkylate as a basefuel offers a high quality, highperformance alternative to conventional Avgas. Such wide boiling rangealkylate basefuels offer a greater choice of basestocks for Avgasformulations and also likely, provide a less expensive basefuel forAvgas compared to the conventional aviation alkylate basefuel.

[0026] In a preferred embodiment, the compositions according to theinvention have good performance in piston-driven aviation engines.Preferably that performance is determined by one or more ratingsincluding MON, Supercharge and Knock Cycles/Intensity at maximumpotential knocking conditions in an aircraft engine. The inventive Avgascompositions preferably have a MON of at least about 94, more preferablyat least about 96 and most preferably at least about 98. Furtherpreferred Avgas compositions have a MON of at least about 99 or morepreferably at least about 100. For example, a preferred MON range may befrom about 96 to about 102. The Supercharge rating is preferably atleast about 130. The inventive Avgas compositions also preferablyminimize, or eliminate, knocking in a piston-driven aircraft engine atmaximum potential knocking conditions. The Knock Cycle rating ispreferably less than (average) 50 per 400 cycles and the Knock Intensityrating is preferably less than 30 per cycle.

[0027] The invention is also directed to a method for preparing an Avgascomposition that involves combining a basefuel, such as a wide boilingrange alkylate, with an additive package. The content and concentrationof the additive package is preferably selected from an inventivenon-linear model that identifies substantially positive or synergisticadditive packages. The method preferably identifies Avgas compositionsthat have good performance in piston-driven aviation engines based onratings of MON, Supercharge and/or Knock Cycles/Intensity.

[0028] The invention is further directed to a method for operating apiston-driven aircraft that involves operating the piston-driven enginewith an Avgas composition made by a composition according to theinvention.

EXAMPLES

[0029] A. Determination of MON

[0030] The MON rating test (ASTM D2700) is conducted using a singlecylinder variable-compression laboratory engine which has beencalibrated with reference fuels of defined octane levels. The sample ofinterest is compared to two reference fuels at standard knock intensityand the octane number of the sample is determined by bracketing orcompression ratio (c.r.) methods. In bracketing, the octane value of thesample is determined by interpolating between two reference fuel octanevalues. In the c.r. method, the octane value of the sample is determinedby finding the compression ratio which duplicates the standard knockintensity of a reference fuel and the octane number is then found in atable of values. Repeatability limits for MON determination at 95%confidence intervals is 0.3 MON for 85-90 MON fuels whilereproducibility limits are 0.9 for 85 MON and 1.1 for 90 MON.

[0031] B. Determination of Supercharge Rating

[0032] The Supercharge rating test (ASTM - D909) determines theknock-limited power, under supercharge rich-mixture conditions, of fuelsfor use in spark ignition reciprocating aircraft engines. TheSupercharge rating is an industry standard for testing the severe octanerequirements of piston driven aircraft. For purposes of thisapplication, “ASTM-D909” is used interchangeably with both “superchargerating” and “performance number.”

[0033] C. Determination of Knock Cycles and Intensity Rating

[0034] For purposes of this application, “Knock Cycle/Intensity ratingtest” and “Lycoming IO-360 tests” are used interchangeably. The KnockCycles/Intensity rating test was performed with a Textron Lycoming10-360 engine (“the Lycoming engine”) on a dynamometer test stand (SeeFIG. 1). Each of the four cylinders of the Lycoming engine was equippedwith a Kistler 6061B piezoelectric transducer. These transducers produceelectric charges proportional to the detected pressures in thecombustion chambers in the Lycoming Engine. The charge was then passedinto four Kistler 5010 charge mode amplifiers which were calibrated sothat output voltage from the amplifiers was equivalent to 20 atmospheresas read by the detector. The voltage was processed through a NationalInstruments NB-A2000 A/D board which reads all four channelssimultaneously at a rate of 250,000 samples per second at a resolutionof 12 bits.

[0035] The data acquisition was facilitated by a computer program (SeeFIG. 2) using National Instruments' Labview programming environment. Thedata acquisition program stores the data from 200 to 400 consecutivefirings from the engine which is typically operated at 2700 rpm, wideopen throttle at an equivalence ratio of about 1.12 and maximum cylindertemperature of just below 500° F. The data is first stored into buffers,then into the Random Access Memory of a Macintosh 8100/80 Power PC andfinally on the hard drive. The raw data files were then backed up ontomagneto-optical discs and post-processed using a Labview program.

[0036] Before storage and processing, data from the individualcombustion chamber firings were passed through a Butterworth 4th orderdigital bandpass filter of 15 kHz-45 kHz range. This is done to isolatefrequencies which could only be significantly excited within thecombustion chamber by a knocking event. The filtered signal was then“windowed” for 3 milliseconds near top dead center of piston travel(compression/expansion stroke). The filtered, windowed signal was thensent through an absolute-value function and integrated to obtain apressure-time-intensity expression of the acoustic energy supplied tothe filter in the 15 kHz-45 kHz band of frequencies detected by thesystem. This value was used to create a scale with which knock intensitywas measured. If the intensity of the integral was found to be greaterthan 20 on this scale, it was determined to be a knocking case and theknocking events per 200 cycles were recorded.

[0037] D. Determination of Non-Linear Models for Identifying AviationFuel Compositions with Desirable MON Ratings

[0038] The effects of various fuel formulations on MON ratings weredetermined using statistically designed experiments. More specifically,the complex relationships between the in-cylinder oxidation chemistriesof the octane boosting additives and the basefuel were investigatedusing face centered cube statistical designs (See, e.g., FIG. 3).

[0039] The statistically designed experiments measured the MON values ofspecific fuel formulations which were combinations of three variables(Manganese level, aromatic amine level duo and oxygenate level) mixedwith a wide boiling range alkylate. The three variables and theirrespective concentration ranges define the x, y and z axes of the cube.(See FIG. 3). The cube faces (surfaces) and the space within the cubedefine all the interaction points for investigation. The three variabletest ranges were 0-10 wt % aromatic amine, 0-0.5 g/gal manganese (Mn)and 0-40 vol. % oxygenate (an alkyl tertiary butyl ether). The manganesemay be provided by a corresponding amount of methyl cyclopentadienylmanganese tricarbonyl (MMT). The two oxygenates tested were methyltertiary butyl ether (MTBE) and ethyl tertiary butyl ether (ETBE). Intotal, four test cubes were designed to measure the numerous fuelcombinations and therefore potentially different chemical oxidationinteractions. The four cube design layouts are listed in Table 4.Aniline and n-methyl aniline were the aromatic amines chosen forcomplete statistical analyses. TABLE 4 Design for Testing CubeIndependent Variables. Cube Number Basefuel Variable 1 Variable 2Variable 3 1 Wide boiling range MMT MTBE Aniline 2 Wide boiling rangeMMT ETBE Aniline 3 Wide boiling range MMT MTBE n-Methyl Aniline 4 Wideboiling range MMT ETBE n-Methyl Aniline

[0040] The MON values were measured at specific points along the threecube axes as well as the cube center point. Multiple measurements weremade at the center point to calculate the MON variation level with theassumption being it is constant over all the test space of the design,i.e. essentially a ten MON number range, 91-101. Polynomial curves werefitted to the data to define equations which describe the three variableinteractions with respect to MON over the entire cube test space. Fromthese equations, the MON performance for all variable combinations canbe predicted within the test space defined by the maximum and minimumconcentration ranges of the variables. Some of the predicted andmeasured MON values have been summarized in Tables 5-8. The remainder ofthe predicted values can be derived from the prediction equations. TABLE5 Predicted MON versus Measured MON for Oxygenate + Aniline Manganese =0 g/gal 0 2 6 10 Aniline wt % wt % wt % wt % Vol. % MON MON MON MON MONMON MON MON MTBE (p) (m) (p) (m) (p) (m) (p) (m)  0 91.5 91.1 93.8 94.697.1  98.6 98.8 10 92.8 95.0 98.0  99.3 20 93.8 93.6 95.8 98.6 98.9 99.6 30 94.4 96.3 98.8  99.6 40 94.7 95.2 96.5 97.0 98.7  99.2 99.0 0 26 10 Aniline wt % wt % wt % wt % Vol. % MON MON MON MON MON MON MON MONETBE (p) (m) (p) (m) (p) (m) (p) (m)  0 92.3 91.1 93.8 95.9 96.8  99.797.6 10 94.6 95.9 98.5 101.1 20 96.0 94.0 97.2 99.4 98.8 101.7 30 96.697.5 99.4 101.3 40 96.3 96.2 97.0 97.2 98.6 100.1 101.1

[0041] TABLE 6 Predicted MON versus Measured MON for Oxygenate + AnilineManganese = 0.5 g/gal 0 2 6 10 Aniline wt % wt % wt % wt % Vol. % MONMON MON MON MON MON MON MON MTBE (p) (m) (p) (m) (p) (m) (p) (m)  0 96.095.3 97.4 97.7 98.9 98.7 99.1 10 97.3 98.5 99.8 99.4 20 98.2 99.1 99.4100.4 99.6 99.7 30 98.9 99.9 100.6 99.7 40 99.2 100.3 100.1 99.6 100.699.3 99.8 0 2 6 10 Aniline wt % wt % wt % wt % Vol. % MON MON MON MONMON MON MON MON ETBE (p) (m) (p) (m) (p) (m) (p) (m)  0 95.5 95.5 95.996.0 96.8 97.6 97.8 10 97.8 98.0 98.5 99.0 20 99.2 97.5 99.3 99.4 100.599.5 30 99.8 99.6 99.4 99.2 40 99.4 98.4 99.1 100.9 98.6 98.0 97.1

[0042] TABLE 7 Predicted MON versus measured MON for Oxygenate +n-Methyl Aniline Manganese = 0.0 g/gal n- Methyl 0 2 6 10 Aniline wt %wt % wt % wt % Vol. % MON MON MON MON MON MON MON MON MTBE (p) (m) (p)(m) (p) (m) (p) (m)  0 92.1 91.1 93.4 94.0 95.0 95.4 94.7 10 92.6 93.795.0 95.0 20 93.2 93.6 94.1 95.0 94.9 94.6 30 93.7 94.5 95.0 94.2 4094.3 95.2 94.8 94.8 95.0 93.9 94.6 n- Methyl 0 2 6 10 Aniline wt % wt %wt % wt % Vol. % MON MON MON MON MON MON MON MON ETBE (p) (m) (p) (m)(p) (m) (p) (m)  0 92.1 91.1 92.8 93.8 94.1 95.4 95.6 10 93.3 93.8 94.695.5 20 94.5 94.0 94.7 95.2 95.9 95.6 30 95.7 95.7 95.7 95.7 40 96.996.2 96.6 96.2 96.2 95.8 96.5

[0043] TABLE 8 Predicted MON versus measured MON for Oxygenate +n-Methyl Aniline, Manganese = 0.5 g/gal n- Methyl 0 2 6 10 Aniline wt %wt % wt % wt % Vol. % MON MON MON MON MON MON MON MON MTBE (p) (m) (p)(m) (p) (m) (p) (m)  0 97.2 97.7 99.4 97.7 96.4 95.9 10 97.7 98.0 97.796.0 20 98.3 98.4 97.7 97.5 95.6 30 98.8 98.8 97.7 95.3 40 99.4 99.198.7 97.7 94.9 95.3 n- Methyl 0 2 6 10 Aniline wt % wt % wt % wt % Vol.% MON MON MON MON MON MON MON MON ETBE (p) (m) (p) (m) (p) (m) (p) (m) 0 96.6 96.3 97.4 95.9 95.5 95.9 10 97.1 96.9 96.4 96.0 20 97.6 97.496.9 97.2 96.5 30 98.2 97.9 97.5 97.0 40 98.7 98.5 97.3 98.0 97.5 98.4

[0044] The equations which describe the three variable (oxygenate,Manganese and aromatic amine) interactions and ultimately predict MONlevels are listed in Table 8A. TABLE 8A MON Prediction Equations TestCube: MTBE/Aniline/Manganese MON = 91.54 + (0.1466 × MTBE) + (8.827 ×Mn) + (1.252 × Aniline) − (0.006492 × MTBE × Aniline) − (0.8673 × Mn ×Aniline) − (0.001667 × MTBE²) − (0.05437 × Aniline²) Test Cube:MTBE/n-Methyl Aniline/Manganese MON = 92.06 + (0.05563 × MTBE) + (10.23× Mn) + (0.7308 × nMA) − (0.009273 × MTBE × nMA) − (0.8220 × Mn × nMA) −(0.04005 × nMA²) Test Cube: ETBE/Aniline/Manganese MON = 92.32 + (0.2730× ETBE) + (6.349 × Mn) + (0.7429 × Aniline) − (0.009016 × ETBE ×Aniline) − (1.058 × Mn × Aniline) − (0.004362 × ETBE²) Test Cube:ETBE/n-Methyl Aniline/Manganese MON = 92.12 + (0.1185 × ETBE) + (17.04 ×Mn) + (0.3317 × nMA) − (0.1306 × ETBE × Mn) − (0.01099 × ETBE × nMA) −(0.8828 × Mn × nMA) + (0.0218 × ETBE × Mn × nMA) − (16.36 × Mn²)

[0045] The predicted MON variability for all four design cubes is acombination of engine measurement, fuel blending and equation fittingvariability. Table 9 shows the MON engine measurement variability interms of standard deviations for the four test cubes. TABLE 9 StandardDeviations for Four Test Cubes. MTBE, Aniline, Mn 0.70 MON ETBE,Aniline, Mn 0.28 MON MTBE, n-Methyl 0.60 MON ETBE, n-Methyl 0.55 MONAniline, Mn Aniline, Mn

[0046] The pooled standard deviations for the four test cubes is 0.614with 18 degrees of freedom. At the 95% confidence limit this results ina variability of 1.83 MON. Variability, as used here, is defined as itis in ASTM MON rating method D-2700--for two single MON measurements,the maximum difference two numbers can have and still be consideredequal. However, variability as used here is neither purely repeatabilitynor reproducibility, but is somewhere between the two definitions. All168 test fuels were blended from the same chemical/refinery stocks andrandomly MON rated by two operators on two MON rating engines over an 8week period. The accuracy and variability for the equation fittingprocess of the MON data is shown in Table 10. TABLE 10 Equation FittingVariability Root Mean Test Cube R² Value Squared Error Average ErrorMTBE + Aniline 91.0 0.82 0.54 ETBE + Aniline 74.5 1.29 0.88 MTBE +n-Methyl Aniline 77.3 0.99 0.70 ETBE + n-Methyl Aniline 81.3 0.81 0.61

[0047] The R² Values are the proportion of variability in the MON thatis explained by the model over the ten octane number range tested. Thefuel blending variability was not quantified but is not expected to be amajor contributor to the overall predicted MON variability.

[0048] The majority of MON results were obtained while the aromaticamines were set in the statistical cube design as aniline and n-methylaniline. Subsequent work was done to determine other potentially highoctane aromatic amines. (See Tables 11-113.) Specific aromatic amineswere substituted into two different blends; 1) 80 vol. % wide boilingrange alkylate+20 vol. % MTBE and 2) 80 vol. % wide boiling rangealkylate+20 vol. % ETBE. The substituted aromatic amines were blended at2.0 wt %. No manganese was added to these blends. The MON results listedin Tables 11-1 3 are average MON of two tests. TABLE 11 MON Values forMethyl Substitutions on Aniline Ring 80/20 vol % Wide boiling 80/20 vol% Wide boiling range alkylate + MTBE range alkylate + ETBE aromaticamine MON dMON* MON dMON* Aniline 96.3 — 97.3 — o-toluidine 94.5 −1.8 95.2 −2.1 m-toluidine 96.8 0.5 97 4  0.1 p-toluidine 96.8 0.5 96.8 −0.5

[0049] TABLE 12 MON Values for di- and tri- methyl substitutions onAniline Ring 80/20 vol % Wide 80/20 vol % Wide boiling range boilingrange alkylate + MTBE alkylate + ETBE aromatic amine MON dMON* MON dMON*Aniline 96.3 — 97.3 — 2,3-dimethyl Aniline 93.8 −2.6 94.2 −3.12,4-dimethyl Aniline 95.0 −1.3 95.2 −2.1 2,5-dimethyl Aniline 93.9 −2.495.3 −2.1 2,6-dimethyl Aniline 93.3 −3.0 93.4 −3.9 3,5-dimethyl Aniline95.7 −0.6 96.7 −0.6 2,4,6-trimethyl Aniline 92.6 −3.8 93.7 −3.6

[0050] TABLE 13 MON Values for Alkyl Substitutions on Aniline's Amine.80/20 vol % Wide boiling 80/20 vol % Wide boiling range alkylate + MTBErange alkylate + ETBE aromatic amine MON dMON* MON dMON* Aniline 96.3 —97.3 — 4-ethyl Aniline 96.1 −0.3 97.5  0.2 4-n-butyl 95.7 −0.6 96.9 −0.5Aniline n-methyl 95.0 −1.3 95.7 −1.6 Aniline n-ethyl Aniline 91.9 −4.491.9 −5.4

[0051] It can be seen from Tables 11-13 that the aromatic amines whichhave a methyl substitution in the ortho- (or the 2 position) on thearomatic ring as well as the n-alkyl substitutions on the amine are noteffective octane boosting additives for these two basefuels. However,the meta- ring position, (positions 3- and 5-) and the para- ringposition, (position 4-) methyl substituted aromatic amines are generallymore effective octane boosting additives for this basefuel with theexception of the p-toluidine in the ETBE/basefuel case. The relative MONincreasing effectiveness of the different alkyl substituted aromaticamines exemplifies the importance of mapping the chemical oxidationreaction routes for the additives of interest relative to the MON testenvironment. Further data from these experiments are shown in FIGS.4-15.

[0052] E. Determination of Non-linear Models for Identifying AviationFuel Compositions with Desirable MON, Supercharge, and KnockCycle/Intensity Ratings

[0053] To better characterize the performance of fuel formulations, theeffects of various fuel formulations on MON, Supercharge and KnockCycle/Intensity ratings were determined using statistically designedexperiments. The subject fuel compositions were combinations of MTBE,aniline and manganese components and the same wide boiling rangealkylate fuel as the previous designs. The three variable test rangesfor these experiments were 20-30 vol % MTBE, 0-6 wt % aniline and 0-0.1g/gal manganese. Anti-knock ratings of MON, Supercharge and KnockCycle/Intensity ratings were measured at least in duplicate.

[0054] Table 14 shows the non-linear interactions of the fuelcomposition components on the Supercharge rating and average KnockingCycles and average Knock Intensity per 400 consecutive engine cyclesdata. The eight fuel formulations shown represent the extremes of theranges tested.

[0055] Statistical analysis shows an interaction between the MTBE andmanganese terms in the equations for supercharge rating but only whenaniline levels are low with respect to the domain tested. There isanother significant interaction for supercharge rating which is that asMTBE increases the interaction between manganese and aniline becomesantagonistic. Also, the data analysis for Knock Intensity contains anantagonistic interaction between MTBE and aniline. The Knocking Cyclesdata demonstrates a three way interaction between the MTBE, manganeseand aniline. TABLE 14 Measured Octane Parameters with respect to FuelFormulation Average Mn Average Knock MTBE (g/ Aniline SuperchargeKnocking Intensity/ (vol %) gal) (wt %) MON Rating Cycles/400 400 200.00 0 95.4 115.5 121 49 20 0.00 6 97.6 140.2 12 32 20 0.10 0 95.6 118.168 40 20 0.10 6 98.0 142.5 4 24 30 0.00 0 96.2 114.1 66 35 30 0.00 698.3 143.9 2 33 30 0.10 0 97.4 133.5 13 33 30 0.10 6 99.3 144.5 2 20

[0056] Because of the above mentioned non-linear fuel compositioninteractions, neither MON nor supercharge ratings when consideredindividually will always predict the knock-free operation of thecommercial Lycoming 10-360 aviation engine. (See Table 15). The KnockingCycle and Knock Intensity data in Table 15 are the average of duplicate400 cycle tests. TABLE 15 Measured Octane Parameters with respect toFuel Formulation (II) Average Supercharge Knocking Cycles Average KnockFuel Number MON Rating /400 Intensity/400 1 98.4 134.9 17 30 2 98.5142.2  0  0 3 96.5 136.1  0  0 4 96.3 115.1 73 35

[0057] The R² values between MON, Supercharge, Knocking Cycles and KnockIntensity are listed in Table 16. TABLE 16 R² values for Knocking Cyclesand Knock Intensity Predictions Combination R² values MON to predictKnocking Cycles* .44 MON to predict Knock Intensity* .38 Supercharge topredict Knocking .64 Supercharge to predict Knock Intensity* .82

[0058] Table 17 includes the references of pure isooctane as well as theindustry standard leaded Avgas 100 Low Lead. For example, pure isooctanehas a MON value of 100 by definition but knocks severely in the Lycoming10-360 at its maximum potential knock operating condition. Addition oftetraethyllead (TEL) to isooctane is required to boost the superchargerating sufficiently high to prevent auto-ignition in a commercialaircraft engine. TABLE 17 Knock Data for Isooctane and Leaded Avgas 100Low Lead Knock Supercharge Knocking Intensity Fuel MON Rating Cycles/400/400 Isooctane 100 100 85 Not Collected 100 Low Lead 105 131.2 0 0

[0059] Using centered & scaled units for the fuel properties ourequation for MON is:

MON=97.75+0.575*MTBE(s)+0.305*Mn(s)+1.135*Aniline(s)−0.485*Mn(s)².

[0060] Converting to actual units yields:

MON=92.95+0.115*MTBE+25.5*Mn+0.3783*Aniline−194*Mn².

[0061] No interactions were statistically significant.

[0062] Using centered & scaled units for the fuel properties ourequation for supercharge (SC) is.

SC=140.008+2.325*MTBE(s)+3.9*Mn(s)+11.715*Aniline(s)+1.89375*MTBE(s)*Mn(s)−2.39375*Mn(s)*Aniline(s)−2.30625*MTBE(s)*Mn(s)*Aniline(s)−8.653*Aniline(s)².

[0063] Converting to actual units yields:

SC=122.72−0.375*NMTE−294.125*Mn+6.628*Aniline+16.8*MTBE*Mn+0.15375*MTBE*Aniline+60.917*Mn*Aniline−3.075*MTBE*Mn*Aniline−0.9614815*Aniline²

[0064] Looking at the equation in centered and scaled units, we see thatthe interaction between MTBE and Mn is synergistic (coefficient samesign as coefficients for individual effects of MTBE *Mn). But, becauseof the presence of the 3-way interaction between MTBE, Mn, and Aniline,the size of the MTBE*Mn interaction actually depends on the level ofaniline. At the low level of aniline, the MTBE*Mn interaction issynergistic, but as the aniline level increases, the MTBE*Mn interactionbecomes less and less synergistic until it becomes basically zero at thehigh aniline level (if anything, it is antagonistic at this point).Thus, there is a synergism between MTBE and Mn, but generally only atlow levels of aniline.

[0065] A similar description can be used for the Mn*Aniline interaction,where the size of this interaction depends on the MTBE level. At lowlevels of MTBE, the Mn*Aniline interaction is essentially zero, but asthe MTBE level increases the Mn*Aniline interaction becomes more andmore antagonistic. Table 18 below illustrates the above concepts. TABLE18 MTBE Aniline Expected (vol %) Mn (g/gal) (wt %) Actual SC PredictedSC SC¹ 20 0.00 0 122.2, 108.7 115.2 20 0.10 0 116.8, 119.4 119.4 30 0.000 113.0, 115.1 111.5 30 0.10 0 132.1, 134.9 132.5 115.7 20 0.00 6 137.6,142.8 138.8 20 0.10 6 142.7, 142.8 142.7 30 0.00 6 143.8, 143.9 144.3 300.10 6 143.9, 145.1 146.5 148.2

[0066] 1—This is the expected SC value if there was no interaction, thatis if the effects of each of the fuel components were additive.

[0067] Using centered and scaled units for the fuel properties ourequation for Knock Intensity (KInt) is:

KInt=26.5−2.138719*MTBE(s)−1.905819*Mn(s)−5.877127*Aniline(s)+2.477696*MTBE(s)*Aniline(s)+2.711142*Mn(s)²+2.780729*Aniline(s)²

[0068] Converting to actual units yields:

KInt=62.9−0.923283*MTBE−146.56206*Mn−7.9423549*Aniline+0.1651797*MTBE*Aniline+1084.4568*Mn²+0.3089699*Aniline²

[0069] Again looking at the equation in the centered and scaled units,we see that the MTBE* Aniline interaction is antagonistic. Also, notethat this interaction does not depend on the Mn level because there isno 3-way interaction in the model. The following Table 19 illustratesthis interaction. TABLE 19 MTBE Mn Aniline Actual Predicted Expected(vol %) (g/gal) (wt %) Knock Int. Knock Int. Knock Int.¹ 20 0.00 0 52.0,48.1, 38.0 44.4 20 0.00 6 36.1, 27.3, 26.0 27.7 30 0.00 0 34.4, 35.335.2 30 0.00 6 25.7, 40.0 28.4 18.5 20 0.10 0 39.4, 40.9, 38.7 40.6 200.10 6 19.0, 28.4, 19.0 23.9 30 0.10 0 37.6, 30.0, 28.0 31.4 30 0.10 621.0, 19.0 24.6 14.7

[0070] 1—This is the expected Knock Intensity value if there was nointeraction, that is if the effects of each of the fuel components wereadditive.

[0071] It should be pointed out that knock intensity values below 20cannot be distinguished from each other, so the antagonistic effect ofthe MTBE*Aniline interaction may not be quite so significant at the highlevel of Mn (since the expected value under the assumption of nointeraction is 14.7 and the actual values were 21.0 & 19.0).

[0072] Using centered and scaled units for the fuel properties, ourequation for number of Knocking Cycles (Cycles) is:

Y=ln(Cycles+1)=1.529878−0.43339*MTBE(s)−0.376319*Mn(s)−1.469152*Aniline(s)+0.368344*MTBE(s)*Mn(s)*Aniline(s)+0.732549*Aniline(s)².

[0073] Converting to actual units yields:

Y=ln(Cycles+1)=4.4331281−0.0130092*MTBE+29.308018*Mn−0.3641767*Aniline−1.4733759*MTBE*Mn−0.0245563*MTBE*Aniline−12.278133*Mn*Aniline+0.4911253*MTBE*Mn*Aniline+0.0813943*Aniline².

[0074] In either case, the predicted number of knocking cycles is equalto e^(Y)−1.

[0075] This variable was analyzed on the natural log (ln) scale becauseit was observed that the variability was a function of mean level.Analyzing the data on the In scale causes the variability to be moreconstant across mean levels, which is necessary for the statisticaltests performed to be valid. Also, since some observations had values ofzero for number of knocking cycles (the natural log of zero cannot becalculated), 1 was added to every observation so that the Intransformation could be used. Thus, 1 must be subtracted from Y above toget back to the original units.

[0076] Because of the presence of the 3-way interaction in the model andno 2-way interactions, the 3-way interaction can be interpreted in 3ways. We could say that there is a synergistic interaction between MTBE& Mn at low levels of aniline and an antagonistic interaction at highlevels of aniline. This description holds for all pairs of fuelproperties.

[0077] The following Table 20 describes the MTBE*Mn interaction beingsynergistic at low levels of aniline and being antagonistic at highlevels of aniline TABLE 20 Expect- Avg. # of Pred. # of ed # of MTBE MnAniline Knocking Knocking Knocking (vol %) (g/gal) (wt %) Cycles CyclesCycles¹ 20 0.00 0 178.5, 93.0, 28.0 63.9 20 0.10 0 78.5, 48.0, 71.5 62.930 0.00 0 56.5, 73.0 56.0 30 0.10 0 17.0, 0.8, 17.0 11.9 55.1 20 0.00 613.0, 15.5, 0.5 6.2 20 0.10 6 0.0, 5.5, 0.0 0.6 30 0.00 6 1.5, 0.5 0.430 0.10 6 1.0, 0.0 0.4 0.0

[0078] 1—This is the expected avg. # of knocking cycles value if therewas no interaction, that is if the effects of each of the fuelcomponents were additive.

[0079] Note that at the high aniline level, the reason for theantagonistic MTBE*Mn interaction is that the number of knocking cyclescannot be reduced to a value lower than zero. Increasing Mn to 0.10lowers the number of knocking cycles to almost zero and increasing MTBEto 30 also lowers the number of knocking cycles to almost zero.Therefore, increasing both Mn and MTBE at the same time cannot reducethe number of knocking cycles any more.

[0080] Using centered and scaled units for the fuel properties ourequation for # of Knocking Cycles is:

Cycles=4.462241−9.166427*MTBE(s)−7.93772*Mn(s)−26.077604*Aniline(s)+8.742241*MTBE(s)*Aniline(s)+8.491223*Mn(s)*Aniline(s)+5.167309*MTBE(s)*Mn(s)*Aniline(s)+24.483337*Aniline(s)².

[0081] Converting to actual units yields:

Cycles=135.2−2.5482718*MTBE+188.15204*Mn−33.803388*Aniline−20.669236*MTBE*Mn+0.2383288*MTBE*Aniline−115.63548*Mn*Aniline+6.8897453*MTBE*Mn*Aniline+2.7203708*Aniline².

[0082] In this case, the only synergistic interaction is between MTBEand Mn at low aniline levels. All other interactions are antagonistic.The MTBE*Mn synergism at low aniline levels and antagonism at highaniline levels is shown below in Table 21. TABLE 21 Expect- Avg. # ofPred. # of ed # of MTBE Mn Aniline Knocking Knocking Knocking (vol %)(g/gal) (wt %) Cycles Cycles Cycles¹ 20 0.00 0 178.5², 84.2 93.0, 28.0²20 0.10 0 78.5, 48.0, 71.5 61.7 30 0.00 0 56.5, 73.0 58.7 30 0.10 017.0, 0.8, 17.0 15.5 36.2 20 0.00 6 13.0, 15.5, 0.5 7.9 20 0.10 6 0.0,5.5, 0.0 0.0 30 0.00 6 1.5, 0.5 0.0 30 0.10 6 1.0, 0.0 8.2 0.0

[0083] 1—This is the expected avg. # of knocking cycles value if therewas no interaction, that is if the effects of each of the fuelcomponents were additive.

[0084] 2—These observations were not included in the analyses.

[0085] Further data from these experiments are shown in FIGS. 16-30.

[0086] The testing and equation fitting variability of the second set ofexperimentally designed demonstrated in Tables 22 and 23. For thepredicted performance parameter listed in Table 22, the 95% totalvariability is a combination of engine measurement and fuel blendingvariabilities. Table 22 also shows the performance parameter enginemeasurement and fuel blending variability in terms of standard deviationand total variability calculated at the 95% confidence limit. TABLE 22Variability Analysis for Second Cube Sets Performance Parameter StandardDeviation 95% Total Variability MON 0.69 2.07 Performance Number 3.9311.73 Knock Intensity 7.04 19.70 Knocking Cycles (In Scale) 1.15 3.27Knocking cycles (linear 18.6 52.60 Scale)

[0087] Total variability, as used here, is defined as it is in ASTMMethods for two single measurements, the maximum difference two numberscan have and still be considered equal. However, variability as usedhere is neither purely repeatability nor reproducibility, but issomewhere between the two definitions. The accuracy and variability forthe equation fitting process of the performance parameters is shown inTable 23. TABLE 23 Equation Fitting Variability for Second Cube SetPerformance Root Mean Squared Parameter R² Value Error Average Error MON76.8 0.63 0.47 Performance 91.2 3.99 2.50 Number Knock Intensity 60.55.40 3.80 Knocking Cycles (in 74.2 0.83 0.60 small “L” Scale) KnockingCycles 89.1 9.30 7.10 (linear Scale)

[0088] Other features, advantages and embodiments of the inventiondisclosed herein will be readily apparent to those exercising ordinaryskill after reading the foregoing disclosure. In this regard, whilespecific embodiments of the invention have been described in detail,variations and modifications of these embodiments can be effectedwithout departing from the spirit and scope of the invention asdescribed and claimed.

What is claimed is:
 1. An unleaded aviation fuel composition comprising:(1) an unleaded basefuel and (2) a substantially positive or synergisticcombination of (a) an alkyl tertiary butyl ether, and (b) an aromaticamine having the formula

wherein R₁, R₂, R₃ and R₄ are hydrogen or a C₁-C₅ alkyl group.
 2. Thecomposition of claim 1, wherein the basefuel is a wide boiling rangealkylate.
 3. The composition of claim 1, wherein the alkyl tertiarybutyl ether is methyl tertiary butyl ether.
 4. The composition of claim1, wherein the alkyl tertiary butyl ether is ethyl tertiary butyl ether.5. The composition of claim 1, wherein the aromatic amine is analine. 6.The composition of claim 1, wherein R₁, R₂, R₃ or R₄ is methyl.
 7. Thecomposition of claim 1, wherein the aromatic amine is n-methyl aniline,n-ethyl aniline, m-toluidine, p-toluidine, 3,5-dimethyl aniline, 4-ethylaniline or 4-n-butyl aniline.
 8. The composition of claim 1, wherein thecomposition further comprises manganese.
 9. The composition of claim 8,wherein the manganese is provided by methyl cyclopentadienyl manganesetricarbonyl.
 10. The composition of claim 1, wherein the compositioncomprises 0.1 to 40 vol % alkyl tertiary butyl ether, 0.1 to 10 wt %aromatic amine and 0 to 0.5 g manganese.
 11. The composition of claim 1,wherein the composition comprises 15 to 32 vol % methyl tertiary butylether,
 1. 5 to 6 wt % aniline and 0 to 0.1 g manganese.
 12. Thecomposition of claim 1, wherein the composition comprises 15 to 32 vol %ethyl tortiary butyl ether, 1.5 to 6 wt % aniline and 0 to 0.1 gmanaganese.
 13. The composition of claim 1, wherein the MON of thecomposition is at least
 94. 14. The composition of claim 1, wherein theMON of the composition is at least
 96. 15. The composition of claim 1,wherein the MON of the composition is at least
 98. 16. A method forpreparing an unleaded aviation fuel composition comprising: (1)selecting a substantially positive or synergistic combination of (a) analkyl tertiary butyl ether, and (b) an aromatic amine having the formula

wherein R₁, R₂, R₃ and R₄ are hydrogen or a Cl-C₅ alkyl group, and (2)combining the combination selected in step (1) with an unleadedbasefuel.
 17. The method of claim 16, wherein the basefuel is a wideboiling range alkylate.
 18. The method of claim 16, wherein the alkyltertiary butyl ether is methyl tertiary butyl ether.
 19. The method ofclaim 16, wherein the alkyl tertiary butyl ether is ethyl tertiary butylether.
 20. The method of claim 16, wherein the aromatic amine isanaline.
 21. The method of claim 16, wherein R₁, R₂, R₃ or R₄ is methyl.22. The method of claim 16, wherein the aromatic amine is n-methylaniline, n-ethyl aniline, m-toluidine, p-toluidine, 3,5-dimethylaniline, 4-ethyl aniline or 4-n-butyl aniline.
 23. The method of claim16, wherein the composition further comprises manganese.
 24. The methodof claim 23, wherein the manganese is provided by methylcyclopentadienyl manganese tricarbonyl.
 25. The method of claim 16,wherein the composition comprises 0.1 to 40 vol % alkyl tertiary butylether, 0.1 to 10 wt % aromatic amine and 0 to 0.5 g manganese.
 26. Themethod of claim 16, wherein the composition comprises 15 to 32 vol %methyl tertiary butyl ether, 1.5 to 6 wt % aniline and 0 to 0.1 gmanganese.
 27. The method of claim 16, wherein the composition comprises15 to 32 vol % ethyl tortiary butyl ether, 1.5 to 6 wt % aniline and 0to 0.1 g maganese.
 28. The method of claim 16, wherein the MON of thecomposition is at least
 94. 29. The method of claim 16, wherein the MONof the composition is at least
 96. 30. The method of claim 16, whereinthe MON of the composition is at least
 98. 31. A method for preparing acomposition comprising combining a wide boiling range alkylate basefueland a synergistic amount of all tertiary butyl ether, an aromatic amineand manganese sufficient to raise the motor octane number of thecomposition to at least
 94. 32. The method of claim 31, wherein thesynergistic amount is sufficient to raise the motor octane number of thecomposition to at least
 96. 33. The method of claim 31, wherein thesynergistic amount is sufficient to raise the motor octane number of thecomposition to at least
 98. 34. A method for operating a piston drivenaircraft which comprises operating the aircraft engine with the aviationfuel composition of claim
 1. 35. A method for operating a piston drivenaircraft which comprises operating the aircraft engine with the aviationfuel composition made by the method of claim 29.